Video Graphic Array (VGA) monitors, which provide 640X480 pixel resolution, are rapidly becoming the defacto standard of the personal computer (PC) industry. In addition, a large industry segment is advocating video display monitors with higher resolutions, such as 800X600 and 1024X768 pixels. In order to support these above-VGA or Super VGA resolutions, the video display monitor must be compatible with the horizontal sync frequency and vertical sync frequency provided by the graphics controller in the PC. Many video display monitors and graphics controllers are compatible with a plurality of video standards including CGA, EGA, VGA and Super VGA, but are built to default to a particular video standard on powerup. Thus, even though both the video display monitor and the graphics controller may be capable of better than VGA resolution, these components lock themselves into VGA video standards at powerup. In order to access the high resolution capabilities of these components, additional software programming is normally required.
MULTISYNC-type video display monitors are often used in display applications because they automatically adjust to the characteristics of the analog video signal supplied by the graphics controller of the PC. Exemplary U.S. Pat. Nos. 4,574,224 and 4,679,091 disclose a variable rate horizontal deflection system for a video monitor and a multiple scanning type television receiver, respectively. MULTISYNC-type video display monitors provide high resolution, which improves the video image, as well as faster scan rates, which reduce flicker.
The increased performance of MULTISYNC-type monitors creates problems which arise when configuring systems, i.e., matching the video display monitor characteristics to the graphics controller characteristics. Existing VGA monitors provide only limited information to the graphics controller about their capabilities. For example, the graphics controller of the computer system can determine if the video display monitor is color or monochrome, as discussed in greater detail below, and in some instances if the video display monitor is compatible with the 8514 video standard, a high resolution interlaced graphics standard. However, there is no method or apparatus available by which the graphics controller can determine the video display monitor's exact resolution or scan rate characteristics.
Video display monitor control circuits are available which adapt the video display monitor to the characteristics of video signals received from the graphics controller. However, none of these systems include circuitry which provides information about the video display monitor's capabilities to a graphics controller. U.S. Pat. No. 4,916,442 to Kim, for example, discloses pre-controller circuitry for controlling vertical amplitude and synchronization signals sent to a video display monitor, where the behavior of the circuitry is based on the frequency and polarity of horizontal and vertical sync signals provided by a computer processor and to the presence or absence of a PGA signal in the input to the precontroller circuitry. This circuitry provides up to three different vertical sync rates and amplitudes as well as logic for distinguishing three different types of IBM-brand video display monitors from one another. U.S. Pat. No. 4,779,132 to MCBeath et al., discloses display monitor circuitry which senses the polarity of horizontal and vertical sync signals and switches the monitor between three different vertical resolutions (350, 400, and 480 horizontal line resolutions). U.S. Pat. No. 4,727,362 to Rackley et al., on the other hand, discloses display monitor circuitry, which detects the polarity of the horizontal and vertical sync pulses sent to the video display monitor and switches the video display monitor between two different graphics modes, corresponding to CGA and EGA modes.